Toner powder comprising particles having a coating of fluorine-doped tin oxide particles

ABSTRACT

A toner powder comprising toner particles which carry on their surface and/or in an edge zone close to the surface fine particles of electrically conductive material consisting of fluorine-doped tin oxide. The fluorine-doped tin oxide particles have a primary particle size of less than 0.2 micrometer and a specific electrical resistance of at most 50 ohms.meter. The fluorine content of the tin oxide is less than 10% by weight, and preferably is from 1 to 5% by weight.

The invention relates to toner powder comprising toner particles whichhave deposited on their surface and/or embedded or partially embeddedtherein fine particles of fluorine-doped tin oxide.

A frequent technical requirement is to provide electrically insulatingmaterials with an anti-static or electrically conductive coating inorder to prevent electrostatic charges from accumulating on the materialor in order to make the material suitable for a specific use. Thecoating must usually satisfy the requirement of having sufficiently highelectrical conductivity under extreme conditions, particularly undervery dry conditions. An additional requirement is frequently that thecoating should be transparent. This latter requirement applies, forexample, to conductive coatings of exposure windows or exposurecylinders of copying machines or original scanners, to transparent filmmaterials and to electrically conductive colored toner powders whichhave been given the required conductivity by the application of anelectrically conductive coating to the surface of the toner powderparticles.

Transparent electrically conductive or anti-static coatings can beformed by applying a metal, metal oxide or metal salt, e.g., gold,chromium, nickel, palladium, indium oxide, stannic oxide orcupro-iodide, in the form of a thin film to the insulating surface, e.g.by sputtering or vapor-coating in a vacuum. These methods, however, areexpensive and in many cases do not give the required good coatingadhesion. Moreover, these coating methods are not always applicable, forexample, they are not usable for giving fine toner powder particles anelectrically conductive layer.

U.S. Pat. No. 4,431,764 describes compositions for forming transparentelectrically conductive coatings comprising a film-forming binder andfinely divided tin oxide having a particle size less than 0.4micrometer, the tin oxide being doped with 0.1 to 20% by weight ofantimony, in the form of Sb₂ O₃ or Sb₂ O₅.

The tin oxide described in that patent can also be used to formconductive layers without the aid of a film-forming binder, by softeningthe (thermoplastic) surface of the area to be coated, softening beingcarried out either by heat or by means of a solvent or solvent vapor,and by covering the softened and thus tacky surface with the tin oxide.For example, electrically conductive toner powder having a specificresistance between 10³ and 10¹⁰ ohms.meter can be made by softening thetoner powder particles in a hot gas stream and bringing the fine tinoxide powder into the gas stream, or by intensively mixing a mixture oftoner powder and fine tin oxide in a ball mill for some time so that thetoner powder particles soften as a result of the frictional heatproduced and the tin oxide particles are deposited on the softenedsurface.

Disadvantages of antimony-doped tin oxide in accordance with U.S. Pat.No. 4,431,764 are that its electrical resistance is still relativelyhigh, and that the dry powder itself is not loose but has fairly poorflow properties, so that it is difficult to divide it finely. Conductivetoner powders made with this tin oxide contain a fairly considerableamount of loose tin oxide or tin oxide insufficiently bonded to thetoner powder, so that considerable soiling of the apparatus in which thetoner powder is used occurs.

The invention provides a toner powder which does not have thedisadvantages just mentioned. The toner powder according to theinvention comprises toner particles which carry on their surface and/orin an edge zone close to the surface particles of fluorine-doped tinoxide.

The doped tin oxide powder used according to the invention consists ofmainly crystalline tetragonal tin oxide which is doped with fluorine andwhich preferably has a primary particle size less than 0.2 micrometerand a specific electrical resistance of at most 50 ohms.meter. Suchfluorine-doped tin oxide powder can be prepared in a process wherein analcoholic solution of a stannic salt is combined with an aqueoussolution of a fluoride, the tin hydroxide is precipitated from theliquid and separated, the separated product is dried and the driedpowder is heated to a temperature of at least 500° C.

The fluorine-doped tin oxide powder differs from the known commerciallyavailable antimony-doped tin oxide powder according to U.S. Pat. No.4,431,764 in that it has a higher electrical conductivity and betterflow properties.

The powder consists mainly of crystalline, tetragonal tin oxidecontaining bonded fluorine (probably in the form of Sn-F bonds), has aprimary particle size of less than 0.2 micrometer and a specificelectrical resistance of not more than 50 ohms.meter. The percentage offluorine in the tin oxide should be at least 1% by weight and ispreferably between 1 and 10%, more particularly between 1 and 5% byweight. These latter products in particular are distinguished by a lowspecific electrical resistance, which is often well below 25 ohms.meter,and is usually between 1 and 15 ohms.meter depending on the preparationconditions.

The resistance of the powder is measured as follows: A cylindricalcontainer having an inside diameter of 17.2 mm, the base of whichconsists of brass having a thickness of 1.5 mm, and the wall, which hasan internal height of 22.9 mm, consists of Teflon having a thickness of9 mm, is filled with an excess of powder. The filling is then compressedby crushing it ten times in a crusher made by Engelsmann A. G., ofLudwigshafen, Germany. This filling procedure is repeated twice. Excesspowder is then wiped off with a ruler and a brass lid having a diameterof 17.2 mm and a mass of 55 g is placed on the column of powder. Thefilled container is placed in a Faraday cage and a 10 volt current(D.C.) is applied between the base and lid. The current intensity ismeasured after about 20 seconds. The measuring procedure (containerfilling and current measurement) is repeated three times, whereafter theaverage current intensity of the three measurements is calculated.

The resistance of the powder follows from the formula: ##EQU1## where:U=the applied voltage (=10 volts)

A=contact area of lid and powder column (=2.32×10⁻⁴ m²)

h=height of powder column (=2.29×10⁻² m)

Ig=average current strength (in amps).

The fluorine content of the tin oxide powder is determinedpotentiometrically by means of an ion-selective fluorine electrode suchas electrode number 60502150 manufactured by Messrs Metrohm A. G.,Herisau, Switzerland. The procedure used for this test is the standardsolution addition method as described in the brochure supplied byMetrohm A. G. and entitled "Messen in der Chemie". The solution in whichthe fluorine analysis is carried out is prepared according to thefollowing procedure. The following materials are successively weighedout into a platinum crucible with a lid, the weight of which has beenaccurately determined: 1 g of sodium potassium carbonate, about 0.05 gof fluorine-doped tin oxide powder and 1 g of sodium potassiumcarbonate. The closed crucible is calcined for 2 hours and, aftercooling, transferred to a 250 ml beaker, the weight of which has beenaccurately determined and which is filled with 30 g of demineralizedwater, 30 g of concentrated hydrochloric acid (specific gravity 1.19)and 10 g of a 2% solution of tartaric acid in demineralized water. Theliquid is heated and stirred until the crucible contents are completelydissolved. After cooling, the pH of the solution is brought to between 4and 8 by the addition of a concentrated potassium hydroxide solution(47% by weight). After the beaker and contents have been accuratelyweighed, the solution is filtered over a folded filter into a plasticbottle. The potentiometric analysis of the fluorine content is carriedout with a liquid containing 40 g of the tin oxide solution describedabove, 40 g of buffer solution and 0.4 g of standard sodium fluoridesolution in demineralized water.

The buffer solution is prepared as follows: 58.4 g of sodium chlorideand 5 g of titriplex IV (Article number 8424 of Merck AG, Darmstadt,Germany) are dissolved in about 600 g of demineralized water, whereafter60.7 g of acetic acid are added to the solution and it is made up toabout 900 g with demineralized water. By the addition of sodiumhydroxide pellets, the pH of the solution is then brought to between 5and 5.5. The mass of the solution is then made up to 1000 g by theaddition of demineralized water.

The concentration of the standard sodium fluoride solution is determinedby measuring, by means of the ionselective fluorine electrode, thepotential of a liquid containing 10 g of the above-described tin oxidesolution and 10 g of buffer solution and calculating the fluorineconcentration by reference to the potential as measured. For thestandard solution the fluorine concentration used is 100 times thecalculated concentration.

The fluorine-doped tin oxide powder is prepared according to theinvention by combining a solution of a stannic salt in a water-misciblealcohol with an aqueous solution of a fluoride and separating theresulting precipitate of tin hydroxide from the liquid. This precipitateis then dried at a temperature not exceeding about room temperature andthen calcined by heating it for some time to a temperature of at least500° C. During calcination, it is heated at least until the fluorinecontent of the powder is at most 10% by weight and is preferably between1 and 5% by weight.

The known stannic salts soluble in water-miscible alcohols can be used,such as stannic sulphate and stannic chloride. Stannic chloride ispreferably used, dissolved in ethanol or methanol. The water-solublefluoride may, for example, be sodium fluoride, ammonium fluoride orstannous fluoride. The latter substance is preferred because itgenerally gives tin oxide powders having a lower resistance than thepowders prepared under otherwise identical conditions from sodium orammonium fluoride.

Calcination of the dried tin hydroxide is carried out by heating the tinhydroxide for some time in air or an inert gas such as nitrogen. In thecase of calcination in air, a condition in which the fluorine content ofthe powder is between 1 and 5% by weight and the resistance has reacheda minimum value is fairly quickly obtained, usually within 15 minutes.If calcination is carried out in nitrogen the process is slower, so thatit can be more satisfactorily controlled to give a product of constantquality. The calcination time can then run up to some hours. Calcinationof the tin hydroxide can be carried out directly after drying of the tinhydroxide or at some time thereafter.

Before the tin hydroxide is calcined it is dried, since a precedingdrying step has a favorable influence on the particle size of the finaltin oxide powder. Drying is carried out under moderate conditions,preferably at room temperature or lower. One attractive drying method isfreeze-drying. Moreover, in the preparation of the tin oxide, it ispossible to promote the formation of fine particles by combining thesalt solutions rapidly and with intensive agitation and adding thestannic salt solution preferably to the fluoride solution.

The fluorine-doped tin oxide powder which may be prepared as describedabove is particularly attractive for the manufacture of electricallyconductive toner powder, in which toner particles containingthermoplastic resin are coated with a thin layer of tin oxide on theirsurface.

Preferably, the tin oxide powders used for this application are thosewhich, measured by the known BET method, have a specific area of atleast 50 m² /g and preferably more than 70 m² /g. The coating is made ina manner known per se by softening the surface of the toner particlesand applying the fine tin oxide particles to the softened surface.Suitable processes for manufacturing the toner powders according to theinvention are described, for instance, in British patent specification1406983 and U.S. Pat. No. 3,639,245.

The good flow properties of the powder according to the invention give agood coating in a short time. The fluorine-doped tin oxide powderaccording to the invention has the advantage over antimony-doped tinoxide of a higher electrical conductivity so that less tin oxide has tobe deposited on the toner particles to achieve the same resistancelevel.

The invention will now be explained with reference to examples, whichare presented as illustrative and not limiting in any way.

EXAMPLE 1 Preparation of Fluorine-Doped Tin Oxide Powder

A solution of 25 ml of stannic chloride in 600 ml of ethanol was rapidlyadded to a solution of 25 g of stannous fluoride in 1500 ml of water at85° C. with intensive stirring. During the addition of the stannicchloride solution the pH of the reaction mixture was kept at about 3.5by dropwise addition of ammonia. After the stannic chloride solution hadbeen completely added, the mixture was brought to pH 7 by the additionof ammonia, whereupon the ethanol was evaporated from the mixture. Theprecipitate was separated from the liquid and dried by freeze-drying.

Two tin hydroxides prepared in the above way were heated under thefollowing conditions directly after drying:

Powder 1: 15 minutes at 500° C. in air

Powder 2: 60 minutes at 500° C. in nitrogen.

The fluorine content and the electrical resistance of the resultingfluorine-doped crystalline tin oxide powders were determined in themanner described hereinbefore.

Result:

Powder 1: 3.6% by weight of fluorine, ρ=1.9 ohms.meter

Powder 2: 1.7% by weight of fluorine, ρ=1.6 ohms.meter

The primary particle size of the two tin oxide powders was less than 0.1micrometer.

Five other tin hydroxides (powders 3 to 7) prepared in theabove-described manner were processed into tin oxide under the followingconditions two months after drying:

Powder 3: 15 minutes at 500° C. in air

Powder 4: 30 minutes at 500° C. in nitrogen

Powder 5: 60 minutes at 500° C. in nitrogen

Powder 6: 120 minutes at 500° C. in nitrogen

Powder 7: 180 minutes at 500° C. in nitrogen.

The resulting tin oxide powders had a primary particle size of less than0.1 micrometer.

The fluorine content and the electrical resistance of these powders wereas follows:

Powder 3: 4.4% by weight of fluorine, ρ=9.2 ohms.meter

Powder 4: 2.2% by weight of fluorine, ρ<1.3 ohms.meter

Powder 5: 3.0% by weight of fluorine, ρ<1.3 ohms.meter

Powder 6: 2.0% by weight of fluorine, ρ=1.3 ohms.meter

Powder 7: 1.2% by weight of fluorine, ρ=1.9 ohms.meter

As a comparison the commercially available antimony-doped tin oxide,Mitsubishi T1, prepared according to U.S. Pat. No. 4,431,764, has aresistance of about 80-100 ohms.meter, a primary particle size less than0.2 micrometer and a specific area of 65-75 m² /g.

EXAMPLE 2

Tin hydroxide was prepared in the manner described in Example 1 exceptthat 25 g of ammonium fluoride was used instead of 25 g of stannousfluoride.

Four tin hydroxides prepared in this way were processed into tin oxidein the manner described below.

Powder 1: Heating in air at 500° C. for 15 minutes directly afterdrying.

Powder 2: Heating in nitrogen at 500° C. for 60 minutes directly afterdrying.

Powder 3: Heating in air at 500° C. for 15 minutes two months afterdrying.

Powder 4: Heating in nitrogen at 500° C. for 60 minutes two months afterdrying.

The fluorine content and the electrical resistance of the resulting tinoxides were as follows:

Powder 1: 4.4% by weight of fluorine, ρ=21.5 ohms.meter

Powder 2: 3.0% by weight of fluorine, ρ=5.5 ohms.meter

Powder 3: 6.2% by weight of fluorine, ρ=33.7 ohms.meter

Powder 4: 4.1% by weight of fluorine, ρ=14.4 ohms.meter

EXAMPLE 3

Two tin oxide powders 1 and 2 were prepared by the method of Example 1,the hydrates of the two powders being heated in nitrogen at 500° C. for60 minutes two months after drying. The hydrate of powder 1 was broughtto 500° C. from room temperature in 15 minutes, whereas the heating-uptime for the hydrate of powder 2 was about 60 minutes. The followingproperties were measured for the resulting fluorine doped tin oxidepowders:

Powder 1: 1.5% by weight of fluorine, ρ=1.2 ohms.meter, specific area=63 m² /g.

Powder 2: 3.9% by weight of fluorine, ρ=1.8 ohms.meter, specific area=100 m² /g.

EXAMPLE 4

The procedure of Example 1 was repeated, however, the hydrate after ithad been separated from the precipitation liquid was washed withdemineralized water having a temperature of 70° C. and not dried untilafter this step. A portion of the hydrate was heated in nitrogen at 500°C. for 60 minutes directly after drying, the heating time to 500° C.being 60 minutes. The resulting tin oxide powder had the followingproperties: 1.1% by weight fluorine, ρ=1.6 ohms.meter, specific area =43m² /g. Another portion of the hydrate was processed in the same way asthe first portion two months after drying. The resulting tin oxidepowder had: 1% by weight of fluorine, ρ=2.5 ohms.meter, specific area=78 m² /g.

EXAMPLE 5 Preparation of a Conductive Toner Powder

400 g of polyester resin derived from fumaric acid and oxypropylatedbispheno A (type Atlac of ICI) were melted and 100 g of magneticallyattractable pigment (Bayferrox made by Bayer AG) were homogeneouslydistributed in the melt. After cooling the solid mass was ground to giveparticles having a particle size of between 10 and 20 micrometers.

100 g of the resulting magnetically attractable resin particles weremixed together with 7 g of fluorine-doped tin oxide (powder 2 of Example3) in a 500 ml ball mill filled with 400 g of glass balls having adiameter between 0.6 and 3 mm. After about 90 minutes, the powder wassifted, the particles with a particle size of between 10 and 20micrometers being isolated. The resulting toner powder had a resistanceof 10⁵ ohms.meter when measured in the manner described hereinbefore.The powder was used in a printer of the type described in FIG. 1 ofEuropean Patent Application No. 0 191 521 (published on Aug. 20, 1986).After 5000 prints there was no appreciable soiling of the developingunit of the printer.

In the same way as described above, a toner powder was prepared using 8g of commercially available antimony-doped tin oxide instead of 7 g oftin oxide powder according to Example 3. Toner powder having aresistance of about 10⁵ ohms.meter was obtained after a mixing time ofabout 180 minutes in a ball mill. When used in the printer according toEuropean Patent Application No. 0 191 521 (FIG. 1), a distinct soilingof the developing unit was found after 5000 prints as a result of tinoxide particles having deposited on the parts thereof.

EXAMPLE 6

A conductive toner powder was prepared by tumbling 100 g of red-coloredtoner powder as described in European Patent Application No. 0 156 408,page 9, lines 33 to 36, and 7 g of fluorine-doped tin oxide (powder 2 ofExample 4) in a ball mill in the manner described in Example 5.According to said European Patent Application, the redcolored tonerpowder is prepared by coating spherical magnetically attractable coresconsisting of carbonyl iron particles completely enveloped with epoxyresin successively with granulates prepared in the following manner.

A first granulate is prepared by melting 362 g of epoxy resin (Epikote1001 of Shell-Netherlands) and maintaining it at a temperature of about130° C., and then adding the following ingredients to the melt withcontinuous mixing:

11 g of maleic anhydride

600 g of titanium dioxide

21 g of Maxillon Brilliant Flavine 10 GFF

6 g of Rhodamine F5GL

The melt is intensively mixed at ±130° C. until a homogeneous mass isobtained. This mass is cooled to room temperature, and the solid mass isground into particles having a size between 1 and 3 μm. A pink-coloredgranulate is obtained.

A second granulate is prepared by melting 91 g of epoxy resin (Epikote1001 of Shell-Netherlands) and then adding the following ingredients tothe melt.:

2.7 g of maleic anhydride

4.5 g of Maxillon Brilliant Flavine 10 GFF (yellow-fluorescent dye)

1.1 g of Rhodamine F5GL (orange-red fluorescent dye)

0.5 g of Flexo-red (red fluorescent dye)

The melt is mixed until a homogenous mass is obtained. After cooling,the solid mass is ground into particles having a size between 1 and 3μm.

After about 100 minutes of tumbling, the toner powder had an electricalresistance of about 10⁵ ohms.meter. There was no appreciable colorchange of the toner powder as a result of coating with tin oxide. Whenused in a printer of the type described in FIG. 1 of European PatentApplication No. 0 191 521, no appreciable soiling of the developing unitof the printer was observed after 5000 prints were made.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be considered as adeparture from the spirit and scope of the invention, and all suchmodifications as would be apparent to those skilled in the art areintended to be included within the scope of the following claims.

We claim:
 1. A toner powder comprising toner particles havingelectrically conductive material on the surface and/or in an edge zoneclose to the surface thereof, wherein the electrically conductivematerial comprises fluorine-doped tin oxide.
 2. A toner powder accordingto claim 1 in which the fluorine-doped tin oxide particles have aprimary particle size less than 0.2 micrometer and a specific electricalresistance of at most 50 ohms.meter.
 3. A toner powder according toclaim 1 in which the fluorine content of the fluorine-doped tin oxide isfrom about 1 to 10% by weight.
 4. A toner powder according to claim 3 inwhich the fluorine content of the fluorine-doped tin oxide is from 1 to5% by weight.
 5. A process for preparing a toner powder according toclaim 1, wherein toner particles comprising thermoplastic resin aresoftened by heating and electrically conductive fine powder comprisingfluorine-doped tin oxide particles is applied to the surface of thesoftened toner particles.
 6. A process according to claim 5, wherein thefluorine-doped tin oxide particles have a primary particle size of lessthan 0.2 micrometer and a specific electrical resistance of at most 50ohms.meter.
 7. A process according to claim 5, wherein the fluorinecontent of the fluorine-doped tin oxide is from about 1 to 10% byweight.
 8. A process according to claim 6, wherein the fluorine contentof the fluorine-doped tin oxide is from 1 to 5% by weight.
 9. A tonerpowder comprising toner particles having electrically conductivematerial consisting essentially of fluorine-doped tim oxide on thesurface and/or in an edge zone close to the surface thereof.